1. Field of the Invention
Embodiments described herein relate to the field of seismic exploration in a marine environment. More particularly, to an apparatus and method of transferring seismic equipment to and from an operations platform and an underwater location.
2. Description of the Related Art
Seismic exploration in deep water typically utilizes seismic sensor devices stored on a first marine vessel that are transferred from the first vessel and placed on or near the seafloor or seabed. These devices are typically referred to as Ocean Bottom Cabling (OBC) or Ocean Bottom Seismometer (OBS) systems, such as Seafloor Seismic Recorders (SSR's). These SSR devices contain seismic sensors and electronics in sealed packages, and record seismic data on-board the devices while deployed on the seabed as opposed to digitizing and transmitting the data to an external recorder while deployed. The recorded data is obtained by retrieving the devices from the seabed to a location on the first vessel and downloading the recorded data from the devices to a recorder while onboard the first vessel.
In typical operation, hundreds or thousands of OBS units are deployed from the first vessel to the seabed from the first vessel. In one conventional method, the OBS units are deployed using a remotely operated vehicle (ROV) tethered to the first vessel. The ROV is lowered below the surface of the water and positioned subsurface. One or more OBS units are placed by the ROV on the seabed at predetermined locations in a linear row, which may be known as a receiver line. When at least one receiver line consisting of a suitable number of the OBS units is formed, a seismic survey may be performed by providing a source signal, such as an acoustic or vibrational signal. Reflected signals from the seabed and underlying structures are recorded on the one or more OBS units. The source signal or “shot” is typically provided by a second marine vessel, which may be known as a gun boat.
In the deployment of the OBS units, the speed at which the OBS units can be deployed is primarily limited to the speed at which the equipment can be towed through the water. Specifically, support equipment for the ROV, such as an umbilical cable and a tether management system (TMS) have large drag coefficients. The drag of these components typically limit the speed of the first vessel. Thus, the number of OBS units that can be deployed or retrieved in a given time period is limited. The deployment time also affects the efficiency of the seismic survey as the second vessel must wait until the at least one receiver line is laid prior to shooting. The first vessel continues laying other receiver lines while the second vessel is shooting, but as shooting is often completed prior to completion of the next receiver line, the second vessel must again wait until the second receiver line is formed.
Therefore, what is needed is a method and apparatus for transferring seismic sensor devices to and from the first vessel and/or the ROV in a manner that maximizes the number of seismic sensor devices deployed and retrieved, and provides a buffer for a second vessel.